The present invention relates to a self-configuring amplifier wherein voltage spikes on the outputs arising from the switching from a single-ended configuration to a bridge configuration and vice versa of a pair of amplifying stages are effectively depressed.
In many applications and primarily in audio reproduction systems, for example in car radios and similar apparatuses that are intrinsically compact because of stringent installation requirements, as well as in portable apparatuses, power dissipation in final amplifying stages, often quadruple in order to drive a pair of loudspeakers (front and rear) for each stereo channel, may create problems of heat balance. For example, four 20 W amplifiers may have a power dissipation of about 4.times.12=48 W and because of the limited space available in certain apparatuses, such a power may be difficult to be dissipated without an increased internal temperature of the apparatus. On the other hand, a relatively high temperature of operation may damage the magnetic tape of cassettes or optical disks (CD), the drives of which are often tightly fitted inside a single apparatus case.
The absence of a double (or symmetrical) supply in many of these apparatuses does not allow the use of highly efficient configurations in terms of power dissipation, as for example a so-called class-G configuration.
With the aim of overcoming these limitations or, notwithstanding these limits, reducing somewhat power dissipation for the same audio power output, particular self-configuring techniques of several integrated amplifiers have been developed for optimizing power dissipation by modifying their configuration in function of the signal level.
Systems of this type are described in the U.S. Pat. Nos. 5,365,188 and 5,444,417 in the name of the same applicant, both of which are hereby incorporated by reference.
As mentioned at the end of the description of the self-configurable double bridge amplifier in these patents, during operation phases in which an amplifier of each pair of amplifiers is configured so as to function as a single-ended amplifier for driving its own load in series with the load of the other amplifier of the other pair, the common mode control loop of each pair of configurable bridge amplifiers is automatically deactivated, according to well known techniques.
An example of a common mode control loop that can be activated during a phase of bridge configuration of the amplifiers is described and illustrated in prior U.S. patent application Ser. No. 793,628.
In FIGS. 4 and 5 of said prior patent application is shown a typical common mode control loop constituted by a voltage divider R6-R7 and a differential amplifier TR suitable to force the maintenance of a constant voltage on a common output node C of the pair of amplifiers A1 and A2. The common mode control loop is activated or deactivated by the configuring switch F. In practice the loop is deactivated when the second amplifier A2 is deactivated and the amplifier A1 drives the load L in a single-ended mode.
A self-configuring, double bridge amplifier, similar to the one described in the prior U.S. Pat. Nos. 5,365,188 and 5,444,417, is depicted in FIG. 1.
The respective common mode control loops of each of the two pairs of amplifiers are also shown in the circuit diagram of FIG. 1. Functionally, each common mode control loop comprises a resistive voltage divider RC1-RC2 (RC3-RC4), a common mode differential amplifier Gm1 (Gm2) and a configuring switch SW.sub.-- CL1 (SW.sub.-- CL2), according to a known technique.
It has been observed that such a common mode control system produces some important drawbacks.
When the system functions with the configuring switches SW.sub.-- F open and SW.sub.-- C closed, that is when the double bridge amplifier is configured for a relatively low output power (low level of the input signal) by the configuring window comparator, the common mode feedback amplifiers Gm1 and Gm2 are practically deactivated by their respective configuring switches SW.sub.-- CL1 and SW.sub.-- CL2 which short-circuit the input terminals of the respective feedback amplifier.
When, upon the rise of the input signal level, the configuring window comparator commands the configuring switches to implement a double bridge structure, that is with SW.sub.-- F closed and SW.sub.-- C open, the relative switches SW.sub.-- CL1 and SW.sub.-- CL2 switch to ground potential. As a consequence, each common mode amplifier, for example Gm1, acts on the respective pair of operational amplifiers OPA+F and OPA-F which compose a single bridge driving a respective load (loudspeaker), so as to bring the voltage (Vc1) of the common node of the resistances RC1 and RC2 to ground potential. In the waveforms shown in FIG. 3a, it may be observed that these transitions of the inverting input of the common mode feedback amplifier cause abrupt voltage variations on the outputs of the amplifiers (instants labelled A, B, C and D in the diagrams) when the node Vc1 switches from a certain potential assumed before the switching to ground potential.
Similar abrupt variations of the voltage would occur also in the case where a disabling switch of the common mode control loop is used at the output of the common mode feedback amplifiers (Gm1, Gm2) instead of using a switch for short-circuiting its inputs as in the scheme shown in FIGS. 4 and 5 of the prior U.S. patent application Ser. No. 793,628.
These abrupt voltage changes, occurring at every transition from a configuration to the other, cause distortions that are hardly recovered by the feedback system of the amplifier because they encompass a range of high frequency harmonics, where the gain of the feedback loop of the amplifier and therefore the reduction of distortion is relatively modest. Another negative effect of these abrupt voltage changes is represented by the consequent electromagnetic disturbance (emi) they may generate.
These problems and limitations of the known systems are overcome by the present invention. It has been found that the disturbances caused by switching enabling and disabling a common mode control loop of a pair of self-configuring amplifiers may be substantially eliminated. This is achieved by implementing a different method of enabling/disabling the common mode control loop. According to the invention, enabling/disabling of the common mode control loop is effected by employing a configuring switch and a storage capacitance for retaining a common mode voltage information during the period in which the configuring switch remains open. Substantially this maintains the voltage of the common mode control node at the same value it had at the last switching instant.
In this way, large and abrupt voltage transitions are prevented in the output signals of the configurable amplifiers.
The elimination of the problems connected with the abrupt voltage variations provoked by the configuring switchings, that is by the alternating enabling/disabling of the common mode control loop, is achieved with a negligible increase of the complexity of the circuit of the system of self-configuring amplifiers as a whole, because the storage capacitances may be very small and therefore easily integratable.